Flexible printed circuit boards such as FPC, TAB and COF are conventionally manufactured as follows: First, a laminate in which a desired circuit pattern is formed is produced by laminating a photosensitive film or coating and drying a photosensitive resin on “polyimide film/adhesive/copper foil”, and exposing and developing the resultant. Thereafter, an insulative protective film (coverlay film) is coated or a thermosetting paste is coated and cured on the surface of the copper circuit pattern formed.
However, as a printed circuit meeting the recent demands of compaction and weight saving of electronic devices, flexible printed circuit boards are attracting attention and their use is now being widened. Especially, with the compaction and densification of packaging by increasing the number of pins in semiconductor packages, thinning the lines constituting the circuits is strongly demanded. Further, in addition to the flexibility which is the intrinsic property of the flexible printed circuit boards, suppleness is now demanded as a new characteristic, and as the resin constituting the paste, polyimide resins, polyamideimide resins and polyamide resins are now used in addition to the conventionally used epoxy resins. However, these resins used in the paste have a rigid structure, and there are problems in that in cases where they are applied to a thin substrate, the substrate after curing largely warps, and the cured film lacks suppleness and has poor flexibility.
The conventional flexible printed circuit boards protected by an insulative protective film (coverlay film) are generally produced by first forming holes at desired sites in the coverlay film made of a polyimide film or the like having an adhesive on one side thereof, and the holed coverlay film are laminated on a flexible printed circuit board by thermal lamination or pressing. To protect the circuits, the surface of the flexible printed circuit boards is usually covered with a polyester resin film, polyimide resin film or the like. However, in cases of thin circuits wherein the width of the wiring is 80 μm or less, and the intervals between wirings are 80 μm or less, it is difficult to completely embed an adhesive in the irregularities between the wirings.
With the progress of miniaturization of the wiring in the flexible printed circuit boards, the method in which the coverlay film after forming the holes at the terminals of the circuit or the joint portions with other parts is subjected to positioning with the flexible circuit board (FCCL) having a formed circuit pattern has problems in that the method has a limited ease of handling and a limited positioning precision, and the production yield is low.
There is a second method wherein the holes are formed only at the prescribed positions in the coverlay film by laser etching, plasma etching or the like, after heat pressing a coverlay film made of a polyimide film or the like having an adhesive on one side thereof with a flexible circuit board. By this method, although the positioning precision is very high, there is a drawback in that holing is time-consuming and the costs of apparatus and running thereof are high.
For solving these problems, a method wherein a photosensitive resin composition is used as the coverlay film which is subjected to lamination; a method wherein a polyimide ink is directly applied by screen printing; and a method wherein a photosensitive coverlay film prepared from a photosensitive resin composition for photoresists is used; are known.
Various pastes using a polyimide resin which is made flexible and of which elastic modulus is decreased have been proposed. However, as a solvent for preparing a varnish or paste, a high boiling nitrogen-containing solvent such as N-methyl-2-pyrrolidone is used, so that curing at a high temperature of not lower than 200° C. is required, which brings about a problem that thermal deterioration of electronic parts occurs. Further, there in also a problem in that in cases where the paste after being coated on a substrate is left to stand for a long time, whitening of the coated film due to moisture absorption and formation of voids occur, and the handling conditions are complicated.
As a method for screen printing with a polyimide ink, a method is known wherein a solution of a partially imidized polyamic acid at a high concentration is coated on a substrate through a template, and the coated film on the substrate is completely imidized (Patent Literature 1). It is necessary to heat the formed coated film at a high temperature of 240° C. to 350° C. In the imidization reaction, the fact that the shrinkage of the formed polyimide resin is large is a big problem in processability, and especially, it is difficult to mold the resin as a protective layer with a precise pattern on a semiconductor wafer or the like. Further, since the solvent used in the ink is NMP, DMF or the like with a high moisture absorption, problems that the polyamic acid is likely to precipitate due to the moisture absorption by the varnish, that whitening occurs in printing, that clogging of the screen occurs, and so on occur, so that continuous printing is difficult.
For solving these problems, polyimide siloxane pastes are disclosed in, for example, Patent Literatures 1 and 2, as a paste using a resin soluble in non-nitrogen-containing polar solvents, which gives a coating with a low warping and flexibility. For decreasing the elastic modulus, expensive diamines having dimethylsiloxane bonds are used as the starting material, which is disadvantageous from the viewpoint of economy. Further, there are problems in that with the increase in the amount of the modifying siloxane, the adhesiveness with the sealing material, solvent resistance and chemical resistance (solder flux resistance), as well as PCT (Pressure Cooker Treatment) resistance are degraded.
Further, Patent Literatures 3 and 4 disclose a solution composition of a soluble polyimide siloxane and epoxy resin. Since the polyimide is solvent-soluble, there is a problem in that the composition has a poor chemical resistance. In addition, there are practical problems in that the composition is likely to dry during screen printing, and as a result, clogging of the screen mesh occurs so that the formation of a pattern becomes very difficult. Further, Patent Literature 5 discloses a soluble polyimide composition containing a diamine having 10 mol % of dimethylsiloxane bonds. Although the coating film after drying made from this composition is excellent in chemical resistance, heat resistance and in adhesiveness with substrates and adhesive sheets, improvements in flexibility and in anti-warping are demanded. On the other hand, Patent Literature 6 discloses in Examples 1 and 2 thereof a soluble polyimide composition using a diamine having 33 mol % of dimethylsiloxane bonds, and in Example 4, a soluble polyimide composition using a diamine having 50 mol % of dimethylsiloxane bonds. The coating films after drying made from these compositions are excellent in the low warping, chemical resistance, heat resistance, flexibility and adhesiveness with substrates and adhesive sheets. However, the compositions are especially poor in ease of handling in printing, when viewed from the point of developing the use as an ink for printing.
As a method for forming a pattern of polyimide coating films for photoresists, the so called photosensitive polyimide method is known wherein a polyamic acid precursor is coated on a substrate; the coated film is exposed and developed to dissolve the exposed regions (positive-type) or the non-exposed regions (negative-type); and the remaining polyamic acid is imidized. However, it is necessary to heat the coated film at a temperature as high as 240° C. to 350° C.
On the other hand, as the polyimide compositions for this use, those containing a polyamic acid and a compound having carbon-carbon double bond which can be dimerized or polymerized by a chemical ray and an amino group, or a quaternary salt thereof (Patent Literature 7); and the compositions containing a polyamic acid and an acrylamides (Patent Literature 8) are known.
Compositions containing a polyimide precursor having carbon-carbon double bond-containing groups, a specific oxime compound and a sensitizer are also known as a representative composition (see Patent Literatures 9 to 5). These compositions have negative photosensitivity wherein the exposed regions remain after developing, and receive reasonable evaluation and are used in practice.
For attaining the above-described object, a photosensitive polyimide composition was proposed (see Patent Literature 12) which comprises a polyimide and a photoinitiator, characterized by comprising a polyimide having carboxyl groups in the side chains, which polyimide is obtained by imidization reaction between one or more acid dianhydrides and one or more diamines, and by further comprising an aminoacrylamide compound as a reaction component to be reacted with the above-mentioned carboxyl groups. Patent Literature 12 also discloses a soluble polyimide in which a diamine containing 1 to 10 mol % of dimethylsiloxane bonds. However, the polyimide is obtained by reacting the acid dianhydride and the diamine in the presence of a catalyst in an organic polar solvent containing as a major component N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide or the like, and they have problems of whitening in printing, clogging of the screen sheet and so on, so that the compositions are especially poor in ease of handling in printing, when viewed from the point of developing the use as an ink for printing.
The present applicant previously filed a patent application (Patent Literature 13) directed to a photosensitive resin composition containing a polyimide block copolymer. However, Patent Literature 13 does not disclose or suggest a photosensitive resin composition which exhibits excellent properties when used as a ink for screen printing.    Patent Literature 1: JP 7-304950 A    Patent Literature 2: JP 8-333455 A    Patent Literature 3: JP 4-298093 A    Patent Literature 4: JP 6-157875 A    Patent Literature 5: JP 2003-113338 A    Patent Literature 6: JP 2003-119285 A    Patent Literature 7: JP 59-52822 B    Patent Literature 8: JP 3-170555 A    Patent Literature 9: JP 61-118423 A    Patent Literature 10: JP 62-184056 A    Patent Literature 11: JP 62-273259 A    Patent Literature 12: JP 2003-345007 A    Patent Literature 13: WO 99/19771